In power supply units for an electrical system of a motor vehicle, claw-pole generators having electrical excitation can be used as electric generators (“dynamo;” normally an alternating-current generator; however, the use of direct-current generators is also known). In this case, a torque driving the generator is applied by the internal combustion engine via a mechanical coupling (for example, a belt). The current propagating through the rotor winding is used as a manipulated variable for regulating the on-board voltage and is specified by an assigned field current controller. The control prevents greatly fluctuating voltage values, for example, that could potentially damage the downstream electrical system, from being supplied by the generator in response to widely varying engine speeds. Normally, a controllable electronic switch (for example, a MOSFET) and a corresponding logic circuit select the setpoint for the current that propagates through the rotor winding.
Rectifier circuits can be used to obtain the DC voltage required for the vehicle electrical system from the polyphase AC voltage of the generator. Bridge rectifiers including diodes or switchable rectifier elements (for example, MOSFETs) controlled by a logic circuit can be used in this case.
One or a plurality of capacitors in the intermediate circuit (DC link capacitors) can be used for smoothing the voltage ripple created by the rectifiers. These capacitors are dimensioned to be sufficiently large in order to achieve a DC voltage that is as free as possible of ripple voltage, even under high energy system loads.
During operation, the vehicle electrical system is electrically powered by the power supply unit. Components of the power supply unit itself, such as the rotor winding, the mentioned logic circuits (for example, the field current controller, the rectifier logic), for example, the mentioned switches or the DC link capacitors that are electrically powered, are also considered within the context of the present invention to be vehicle electrical system components that are to be energized.
If the generator is not in operation, the vehicle electrical system is powered by a vehicle battery. During initial start-up of the power supply unit, the rotor winding, the logic circuits, the electronic switches, the DC link capacitors, etc., are fed by the vehicle battery. Problematic here, in particular, is the feeding of the DC link capacitors, for the following reasons.
Since leakage currents across the DC link capacitors are to be avoided upon standstill of the generator, they are typically isolated from the vehicle electrical system in such a case. Therefore, high enough currents occur briefly upon switching of the DC link capacitors into the vehicle electrical system, until the DC link capacitors are charged. These currents can lead to damage to individual or to a plurality of components in the vehicle electrical system. However, a suitable charge state must be ensured for the DC link capacitors to enable the power supply unit to be used.
Possible options for limiting the inrush current of the DC link capacitors include the series connection of an ohmic or inductive load, clocking of the voltage present at the DC link capacitors, the series connection of a current-regulating transistor, an adapted dimensioning of all components of the vehicle electrical system, to prevent damage caused by the high charging current, and/or using a DC voltage converter to regulate the charging current and the charging voltage.
In practice, however, the possible options for operating such power supply units prove to be somewhat complicated, expensive and/or ineffective. Therefore, there continues to be a need for improvements.